Fuel injector method and apparatus

ABSTRACT

One spring and two spring embodimenls of a fuel injection having a nozzle body and needle valve with cooperating inner and outer metering rings providing a metering passage which (a) meters fuel at a reduced rate during an initial increment of valve lift, (b) assists in maintaining fuel pressure at the valve seat to reduce fuel dribble and cavitation erosion during a corresponding last increment or valve closure and (c) dampens secondary pressure waves to prevent secondary fuel injection.

RELATED APPLICATION

This application is a continuation in part of pending U.S. applicationSer. No. 500,714, filed Mar. 28, 1990.Iadd., now U.S. Pat. No.5,020,500, .Iaddend.and entitled "Hole Type Fuel Injector And InjectionMethod".

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates generally to diesel engine fuel injectionand relates more particularly to method and apparatus for shaping therate of fuel injection

A principal object or the present invention is to provide new andimproved method and apparatus in a fuel injector for reducing orregulating the rate or fuel injection during an initial stage ofinjection.

Another object of tile present invention is to provide new and improvedmethod and apparatus in a fuel injector for injecting an initial reducedcharge for pre-injection.

A further object of the present invention is to provide new and improvedmethod and apparatus in a fuel injection for metering fuel during aninitial stage or injection.

A further object of the present invention is to provide new and improvedmethod and apparatus in a fuel injection for assisting in maintainingfuel pressure at the injection valve seat until valve closure to reduceor eliminate secondary fuel injection, end or injection fuel dribble andcavitation erosion at the valve seat and adjacent area.

A further object of the present invention is to provide a new andimproved two stage fuel injector having a regulated or reduced rate orfuel injection during a first stage of injection. In accordance with thepresent invention, the two stage fuel injector may employ one or two (ormore) valve closure springs. In the two spring embodiment, only onespring is effective when the injector needle valve is closed and as theneedle valve is opened to a predetermined intermediate position. Bothsprings are effective as the needle valve is opened from thatintermediate position to its fully open position. In a single springembodiment, a single spring is effective when the needle valve is closedand as the needle valve is opened to its fully open position. In bothversions, during a first stage or needle valve operation, fuel rateshaping is provided in a manner which does not rely on fuel meteringbetween the needle valve and its valve seat and which is substantiallyinsensitive to slight variations in needle valve lift.

A further object of the present invention is to provide a new andimproved fuel injector which fulfills one or more or the foregoingobjects of the present invention and which can be economicallymanufactured on a mass production basis.

Other objects of the present invention will be in part obvious and inpart pointed out more in detail hereinafter.

A better understanding of the invention will be obtained from thefollowing detailed description and accompanying drawings of preferredembodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a longitudinal section view, partly broken away and partly insection, of a single spring fuel injector incorporating an embodiment ofthe present invention;

FIG. 2 is a longitudinal section view, partly broken away and partly insection, of a two spring fuel injector incorporating another embodimentof tile present invention;

FIG. 3 is an enlarged longitudinal section view, partly broken away andpartly in section, of similar parts of the nozzle body and nozzle needlevalve of the fuel injectors of FIG. 1 and FIG. 2;

FIG. 4 is an enlarged longitudinal sectional view, partly broken awayand partly in section, of the nozzle body and needle valve of FIG. 3,showing the relationship of inner and outer metering rings and meteringedges of the nozzle body and needle valve when the needle valve isclosed; and

FIG. 5 is a graph showing the relationship of needle valve lift and timeduring an exemplary fuel injection cycle of the fuel injector of FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENTS

In tile drawings, like numerals are used to represent the same or likeparts or like functioning parts. FIGS. 1 and 2 show two exemplary fuelinjectors 10, 11 which incorporate embodiments of the present invention.Each injector 10, 11 comprises an elongated nozzle body 12 with anelongated valve bore 14 and an elongated nozzle needle valve 16 axiallyreciprocable within the valve bore 14. In injector 10, the nozzle body12 is formed as one piece, whereas in injector 11, the nozzle body 12comprises an upper, elongated body subassembly 84 and a lower elongated,body part 86 having an outer diameter substantially less than that ofthe upper body subassembly 84. The nozzle body 12 of each injector 10,11 has a lower end tip 20 coaxial with and enclosing the lower end ofthe valve bore 14. The nozzle body 12 of each injector 10, 11 has aninternal, upwardly facing, coaxial conical surface 18 providing anannular needle contact area or valve seat 19 immediately above thenozzle tip 20. In each injector 10, 11, the needle valve 16 has a lowerconical end with approximately line contact with the conical surface 18when the valve is closed.

In each injector 10, 11, one or more small diameter spray holes 22 areprovided below the valve seat 19 in the end tip 20. In the alternative(not shown), one or more spray holes 22 may be provided in the conicalsurface 18 below tile valve seat 19. In a conventional manner, the sprayholes 22 provide for spraying small droplets of fuel for combustion. Thenumber, diameter and exact location of the spray holes 22 are selectedfor each application.

Injector 10 has a single valve closure spring 38 whereas injector 11 hastwo valve closure springs 80, 82. In injector 10, the single coilcompression spring 38 is mounted above the needle valve 16 to constantlyurge the needle valve 16 downwardly to its closed position. In injector11, the first or upper coil compression spring 80 is mounted above theneedle valve 16 to constantly urge the needle valve 16 downwardly to itsclosed position via a spring seat 87 and an intermediate pin 88. Thesecond or lower coil compression spring 82 is effective, also via theintermediate pin 88, to urge the needle valve 16 downwardly as theneedle valve 16 is lifted above a predetermined intermediate position.

In the single spring injector 10, a shim 39 is employed to precisely setthe preload of the valve spring 38 and thereby precisely establish thevalve opening pressure (i.e., the pressure at which the needle valve 16begins to lift off the valve seat 19). An adaptor plate 40 mounted onthe nozzle body 12 serves as a stop engageable by an upper guide 30 ofthe needle valve 16 to limit valve lift.

In the two spring injector 11, the upper spring seat 87 and theintermediate pin 88 are mounted between the needle valve 16 and anexternally threaded, central stop 92. The stop 92 is adjustable to setthe maximum valve lift. A first, externally threaded spring seat 90 isadjustable to precisely set the preload of the upper spring 80 andthereby precisely establish the valve opening pressure. A second,externally threaded spring seat 96 is adjustable to precisely set thepreload of the lower spring 82. With the needle valve 16 closed, a lowerspring seat 98 of the lower spring 82 rests on a separate annular washeror shim 100. When the needle valve 16 is lifted to a predeterminedintermediate position having a predetermined intermediate liftestablished by the thickness of the annular shim 100, the intermediatepin 88 engages tile lower spring seat 98 of the lower spring 82. Thatpredetermined intermediate lift preferably is slightly less thanone-half the maximum valve lift.

Injectors 10, 11 are hole type injectors. In each injector, the needlevalve 16 has a predetermined maximum lift which is preferably within theusual range of maximum lift of 0.008 to 0.016 inch of such hole typeinjectors.

Apart from the different effects provided by the different springmechanisms employed in the two injectors 10, 11, even though theinjectors 10, 11 are otherwise structurally different, both injectors10, 11 provide the same general type of two stage valve operationhereinafter described. And tile following description concerning the twostage operation is equally applicable to both injectors 10, 11 exceptwhere otherwise indicated.

The nozzle body 12 has upper and lower, coaxial valve guides or rings26, 28 which cooperate with upper and lower, coaxial guides or rings 30,32 of the needle valve 16 to guide the reciprocal movement of the needlevalve 16. The upper valve guide 26 is located at the top of the nozzlebody 12 and the lower valve guide 28 is spaced below the upper valveguide 26 and above the . valve seat 19. An upper annular fuel chamber 34surrounding the needle valve 16 is provided between the upper and lowervalve guides 26, 28. A lower annular fuel chamber 36 surrounding theneedle valve 16 is provided between the lower valve guide 28 and valve:seat 19.

The diameter of the upper guide 30 of the needle valve 16 is larger thanthe diameter of the annular valve seat 19 to provide a differential areafor hydraulically lifting the needle valve 16 from the valve seat 19 forfuel injection. The needle valve 16 is periodically actuated by highpressure pulses of fuel supplied to the upper annular chamber 34 via aradial port 41 in the nozzle body 12 (FIG. 2) or one or more internalfuel passages 42 in the nozzle body 12 (FIG. 1). As hereinafter morefully described, each high pressure pulse acts on the differential areabetween the upper guide 30 and valve seat 19 to open the needle valve 16and to supply fuel for fuel injection through the spray holes 22.

In a hole type nozzle, in most applications the high pressure pulsestypically have a maximum pressure within a range of 4,000 to 17,000 psi.That maximum pressure and the valve opening pressure are functions ofthe spring characteristics and preload setting of each valve, closurespring (i.e., spring 38 of injector 10 and springs 80, 82 of injector11) and the shape of the high pressure pulse. In a single springinjector, the valve opening pressure typically is within the range of2,800 to 5,000 psi. In a two spring injector, the valve opening pressuretypically is within the range of 2,500 to 3,000 psi. The pressurerequired to raise the needle valve from in predetermined intermediateposition against the preload of the second spring 82 in addition to thebias of the first spring 80 typically is within the range of 3,400 to5,800 psi.

The lower guide 32 of the needle valve 16 cooperates with the lowerfixed valve guide 28 to restrict or throttle fuel flow between the upperand lower fuel chambers 34, 36 during part of the reciproeable movementof the needle valve 16. Regulation is provided during an initial upwardincrement of travel and a corresponding last downward increment oftravel of the needle valve 16. That increment is preferably within therange of approximately 0.004 to 0.008 inch or approximately one-half themaximum lift of the needle valve 16.

The lower guide 32 of the needle valve 16 has upper and lower spacedsections 50, 52 with outer cylindrical surfaces. The lower section 52has three equiangularly spaced, axially extending flats 54 providingaxial passages for unrestricted fuel flow. A conical surface 56, incombination with the flats M, provides a peripheral annulus between thespaced sections 50, 52 for connecting the upper ends of the three axialpassages.

The lower part of the upper section 50 forms an inner metering ring 60that is received within an outer metering ring 62 formed by the lowerfixed guide 28 when the needle valve 16 is seated. The inner meteringring 60 is formed by an external cylindrical meteting surface having alower circular meteting edge 64. The outer, fixed metering ring 62 isformed by an internal cylindrical metering surface having an uppercircular metering edge 66. Each metering edge 64, 66 is a sharp edgeformed in the shown embodiments by the respective cylindrical meteringring 60, 62 and adjacent perpendicular shoulder. A clearance passage 68having a radial clearance b is provided between the two opposingcylindrical metering rings 60, 62. The diametrical clearance between thetwo metering rings 60, 62 in each of the shown embodiments is preferablywithin the range of 0.0003 to 0.0006 inch.

The lower guide section 52 is provided to maintain the concentricity ofthe inner and outer metering rings 60, 62. For nozzles which do not needa lower guide section 52 for that purpose, the lower guide section 52and intermediate conical surface 56 may be excluded and the axial lengthof the lower valve guide 28 may be reduced accordingly.

The inner and outer meteting rings 60, 62 cooperate to regulate flowbetween the upper and lower chambers 34, 36 during part of the upwardand downward movement of the needle valve 16. How metering or throttlingoccurs during an initial increment of needle valve lift and acorresponding last increment of needle valve closure. For example, withthe valve closed as shown in FIGS. 3 and 4, if the axial overlap a ofthe metering edges 64, 66 is 0.006 inch (i.e., metering tings 60, 62have an axial width or overlap a of 0.006 inch), the annular meteringrings 60, 62 cooperate to regulate flow during the initial upward andlast downward increments of movement of the needle valve 16 of 0.006inch. As described, the meteting edges 64, 66 preferably are coaxial,circular edges and the metering rings 60, 62 are formed by cylindricalsurfaces. In the alternative (not shown), one or both of the meteringrings 60, 62 may have a different shape to provide a more gradualtransition between regulated and non-regulated conditions as the needlevalve 16 reciprocates.

Prior to valve opening, the pressure in the lower chamber 36 isessentially the same as that in the upper chamber 34. That is so, evenduring a rapid increase in pressure at the beginning of a high pressurevalve operating pulse, because, with the needle valve 16 closed, onlyextremely little flow through the clearance passage 68 is required toequalize the pressure between the upper and lower chambers 34, 36.However, as the needle valve 16 lifts off the valve seat 19 and fuelflows through the clearance passage 68 and spray holes 22, the lowerchamber pressure will be less than the upper chamber pressure due tofuel throttling or metering provided by the clearance passage 68.Accordingly, at any specific upper chamber pressure, the net hydraulicopening bias on the needle valve 16 is less with the needle valve 16open than closed mid less than it would be if there were no restriction.Consequently, because of the restriction, a higher upper chamberpressure is required to open the needle valve 16 further after it isinitially opened. Further valve opening is therefore slowed or delayedfor a short but meaningful period during which the rate of fuelinjection is metered or throttled by the clearance passage 68.

Thus, needle valve operation and fuel injection occur in two stages: afirst stage of partial needle valve opening during which there is aregulated or reduced rate of fuel injection and a second stage ofunthrottled fuel injection. The first stage may have two distinctphases. During a first hfitial opening phase, as the upper chamberpressure rises above the needle valve opening pressure, the needle valvemay modulate or dither briefly between closed and partly open positions.Valve modulation continues during a succeeding second phase after theupper chamber pressure reaches a level sufficient to keep the needlevalve 16 from closing. In the single spring injector 10, second phaseneedle valve modulation continues until the total needle valve openingforce produced by the different fuel pressures in the upper and lowerchambers 34, 36 is sufficient to propel the needle valve 16 upward toits fully open position. A representative fuel injection cycle of thesingle spring injector 10 is illustrated in FIG. 5. In the two springinjector 11, second phase needle valve modulation continues until thetotal valve opening force is sufficient to lift the needle valve 16 toits predetermined intermediate position where the pin 88 engages thelower spring seat 98 of the second spring 82. After a short delay untilthe total needle valve opening force is sufficient to overcome thepreload of the second spring 82, the needle valve 16 is propelled to itsfully open position. Thus, this short delay adds a third phase to thefirst stage of fuel injection.

The diameter of the lower guide 32 is selected to provide the desiredvalve modulation. At one extreme, if the diameter of the lower guide 32is less than or equal to the diameter of the valve seat 19, there willbe no first stage valve modulation. Instead, in the single springinjector 10, the needle valve 16 will be propelled to its fully openposition in a single step. In the two spring injector 11, the needlevalve 16 will be propelled initially to its predetermined intermediateposition where the second spring 82 becomes effective. After the shortdelay described above, the needle valve 16 will be propelled to itsfully open position. At the other extreme, if the diameter of the lowerguide 32 is equal to or greater than the diameter of the upper guide 30,in both injectors 10, 11, the needle valve 16 will dither or fluctuatebetween closed and partly open positions and never fully open. Althoughneedle valve operation provided by one of those extreme conditions maybe desirable in certain applications, in general the diameter of thelower guide 32 should lie in a central range between the diameter of thevalve seat 19 and upper guide 30.

The two stage valve operation is affected by the pressure/time curve orshape of the high pressure fuel pulse supplied to tile upper fuelchamber 34. For any given fuel injection system, the pulse sixape varieswith engine speed. At higher engine speeds, the pressure of the suppliedhigh pressure pulse increases more rapidly, thereby giving less time foreffective first stage operation to occur. As a result, in the singlespring injector 10, first stage valve operation typically is morepronounced at lower RPM. In the two spring injector 11, first stageoperation can be achieved throughout the desired engine speed range byproper selection of the intermediate valve lift and by employing springs80, 82 with an appropriate preload and spring rate.

Certain nozzle dimensions or parameters are established for eachapplication to provide the desired two stage and two phase operation.For a typical automotive diesel engine application (e.g., a fourcylinder, two liter, engine with injectors which directly inject acharge having a maximum volume of approximately 40 mm³ and which areoperated by high pressure pulses having a maximum pressure, which varieswith engine speed, in the range from 5,000 to 14,000 psi), the nozzleparameters and their preferred nominal dimensional ranges are asfollows:

    ______________________________________                                                             Nominal Dimemsional                                      Parameter            Range                                                    ______________________________________                                        Diameter of upper valve guide 26                                                                   0.150 to 0.180 inch                                      Diameter of lower valve guide 28                                                                   0.098 to 0.160 inch                                      Diametrical clearance 68                                                                           0.0003 to 0.0006 inch                                    Diameter of valve seat 19                                                                          0.079 to 0.104 inch                                      Metering ring width (edge overlap) a                                                               0.004 to 0.006 inch                                      Maximum valve lift   0.008 to 0.012 inch                                      ______________________________________                                    

In the typical automotive diesel engine application described above, itis generally desirable to inject approximately the first 5 mm³ of fuelat n reduced rate to reduce combustion noise and nitrous oxideemissions. Optimum dimensions within the ranges given above areestablished to achieve that level of first stage injection. In otherdiesel engine applications, the optimum dimensions may be outside theranges given.

The axial position of the metering rings 60, 62 relative to the valveseat 19 can affect the two stage operation. In general, it is believedthat the meteting rings 60, 62 should be located closer to the valveseat 19 than to the upper guides 26, 30 to reduce the volume of thelower fuel chamber 36 and thereby increase the responsiveness of theneedle valve 16 to the metered rate of flow through the clearancepassage 68.

As described, the cooperating inner and outer metering rings 60, 62provide fuel throttling and therefore fuel rate shaping during the Firststage of valve operation. First stage fuel regulation is provided in amanner which is substantially insensitive to valve lift since firststage fuel regulation does not rely on fuel metering between the needlevalve 16 and valve seat 19. More effective and consistent rate shapingis thereby achieved.

In the two spring injector 11, tirst stage valve operation can beextended to higher speeds and otherwise modified or enhanced as desired.For example, the second spring 82 is effective at an intermediateposition having a predetermined intermediate valve lift of 0,004 inch(for use in combination with a metering ring width (edge overlap) a of0.006 inch and a total valve lift of 0.012 inch). During first stagevalve operation, the needle valve 16 is temporarily held at thatpredetermined intermediate position by the preload of the second spring82.

During second stage valve operation (for designs employing either one ortwo needle valve closure springs), the rate of fuel injection is notaffected by the metering rings 60, 62. Also, the transition between thefirst and second stages, during wtdch the cooperating metering rings 60,62 have varying transitional affect, is extremely quick. During thefirst stage, valve behavior and the rate of fuel injection aredetermined primarily by the rate of fuel flow between tile meteringrings 60, 62. During the second stage, the needle valve 16 is quicklypropelled to and then temporarily held at its fully open position. Thewidth (edge overlap), diameter and configuration of tile metering rings60, 62, the spring rate and preload of each valve spring and theintermediate valve position are predetermined for each nozzleapplication to sitape that two stage valve operation as desired.

The metering rings 60, 62 also affect fuel flow during valve closure.During the last increment of valve closure, the two rings 60, 62cooperate to restrict fuel flow between the upper and lower chambers 34,36. Also, the lower guide 32 of the needle valve 16 serves as a pump topressurize fuel in the lower chamber 36 if, as preferred, the innermetering ring 60 has a diameter larger than the valve seat 19. Thatpumping actlon is affected by the design parameters and other factorsdiscussed above. By that pumping action, the fuel pressure at the sprayhole(s) 22 and valve seat 19 is maintained at a higher pressure thanotherwise until the needle valve 16 is completely closed. The higherpressure helps eliminate or reduce fuel dribble from the spray hole(s)22 and helps eliminate or reduce cavitatlon within the lower fuelchamber 36 by helping both to collapse and to prevent vapor cavitieswhich typically form at or near the valve seat 19 during valve closure.Cavitatlon erosion at or adjacent the valve seat 19 is thereby reducedor eliminated. In addition, tile clearance passage 68 dampens thetransmission, from the upper chamber 34 to the lower chamber 36, of anysecondary pressure waves caused by reflection of the injection pulse andfollowing each injection event. Such dampening eliminates undesirable"secondary" fuel injection and further minimizes cavitation within thelower fuel chamber 36 and thus minimizes cavitation erosion at and nearthe valve seat 19.

The disclosed exemplary fuel injectors 10, 11 are hole type fuelinjectors and are designed to be employed in fuel systems in which aremote high pressure pump is utilized to supply high pressure fuelpulses to the fuel injectors 10, 11 via high pressure fuel lines. Thepresent invention is also readily adaptable to other types of fuelinjectors, for example unit injectors employing a high pressure pump aspart of each injector assembly and pintle type fuel injectors. Inaddition, as will be apparent to persons skilled in the art, othermodifications, adaptations and variations of the foregoing specificdisclosure can be made without departing from the teachings of thepresent invention.

I claim:
 1. A fuel injector comprising a nozzle body with an elongatedvalve bore, an annular valve seat and longitudinally spaced, coaxial,upper valve guide and lower valve ring above the valve seat; anelongated nozzle needle valve in the valve bore having longitudinallyspaced, coaxial, upper guide and lower ring which cooperate with theupper valve guide and lower valve ring respectively of the nozzle bodyto provide axial movement of the needle valve within the valve borebetween a lower closed position in engagement with the valve seat and anupper fully open position with a predetermined maximum lift; the nozzlebody having hole means connected to the valve bore below the valve seatfor injection of fuel; the nozzle body providing an upper fuel chambersurrounding the needle valve between the upper valve guide and lowervalve ring and a lower fuel chamber surrounding tile needle valvebetween the lower valve ring and valve seat; two stage valve closurespring means biasing the needle valve downwardly into engagement withthe valve seat, including first stage spring means holding the needlevalve in its closed position and biasing the needle valve downwardly asit is lifted upwardly from its closed to its fully open position andsecond stage spring means biasing the needle valve downwardly as it islifted upwardly from a predetermined intermediate position with apredetermined intermediate lift to its fully open position; the upperguide of the needle valve having a greater diameter than the valve seatto provide a differential area for hydraulically opening the needlevalve against the bias of the valve closure spring means; the upper fuelchamber being connected to receive periodic high pressure pulses of fuelfor opening the needle valve against the bias of the spring means andfor supplying fuel for fuel injection through the hole means; the lowervalve ring forming an outer metering ring with an internal, annularmetering surface with an upper metering edge; the lower ring of theneedle valve forming an inner metering ring with an external annular,metering surface with a lower metering edge; the inner metering ring,with the needle valve in its closed position, being received within theouter metering ring with the inner ring metering edge below the outerring metering edge by a predetermined axial overlap substantially lessthan said predetermined maximum lift and slightly more than saidpredetennined intermediate lift and with a predetermined annularclearance between the inner and outer metering surfaces providing ametering passageway to regulate fuel flow between the upper and lowerfuel chambers during an initial increment of upward movement of theneedle valve from its closed position and to its said intermediateposition and to regulate the pressure in the lower fuel chamber duringthe corresponding last increment of downward movement of the needlevalve.
 2. A fuel injector according to claim 1 wherein the fuel injectoris a hole type injector in which the nozzle body has a nozzle tip belowthe needle valve enclosing the lower end of the valve bore and said holemeans comprises one or more spray holes connected to the valve borebelow the valve seat.
 3. A fuel injector according to claim 1 whereinsaid axial overlap is approximately one-half said predetermined maximumlift.
 4. A fuel injector according to claim 1 wherein said axial overlapis greater than said predetermined intermediate lift in the range of0.001 to 0.005 inch.
 5. A fuel injector according to claim 1 whereinsaid clearance is a diametrical clearance in the range of 0.0003 to0.0006 inch.
 6. A fuel injector according to claim 1 wherein said axialoverlap is no greater than approximately 0.008 inch.
 7. A fuel injectoraccording to claim 1 wherein the outer ring metering edge is circular.8. A fuel injector according to claim 1 wherein the inner ring meteringedge is circular.
 9. A fuel injector according to claim 1 wherein theinner ring metering surface is cylindrical.
 10. A fuel injectoraccording to claim 1 wherein the outer ring metering surface iscylindrical.
 11. A fuel injector according to claim 1 wherein the innermetering ring has a diameter greater than that of the valve seat andless than that of the upper guide of the needle valve.
 12. A method offuel injection with a fuel injector comprising a nozzle body with anelongated valve bore, an annular valve seat and longitudinally spaced,coaxial, upper valve guide and lower valve ring above the valve seat; anelongated needle valve in the valve bore having longitudinally spaced,coaxial, upper guide and lower ring which cooperate with the upper valveguide and lower valve ring respectively of the nozzle body to provideaxial movement of the needle valve within the valve bore between a lowerdosed position in engagement with the valve seat and an upper fully openposition having a predetermined maximum lift; the nozzle body havinghole means connected to the valve bore below the valve seat forinjection of fuel; the nozzle body providing an upper fuel chambersurrounding the needle valve between the upper valve guide and lowervalve ring and a lower fuel chamber surrounding the needle valve betweenthe lower valve ring and valve seat; closure spring means biasing theneedle valve downwardly into engagement with the valve seat, includingfirst stage spring means holding the needle valve in its closed positionand biasing the needle valve downwardly as it is lifted upwardly fromits closed to its fully open position and second stage spring meansbiasing the needle valve downwardly as it is lifted upwardly from apredetermined intermediate position with a predetermined intermediatelift to its fully open position; the upper guide of the needle valvehaving a greater diameter than the valve seat to provide a differentialarea for hydraulically opening the needle valve against the bias of thevalve closure spring means; the upper fuel chamber being connected toreceive high pressure pulses of fuel for opening the needle valveagainst the bias of the spring means and for supplying fuel for fuelinjection tiaough the hole means; the method comprising the steps ofproviding a predetermined fuel metering passage between the lower ringsof the nozzle body and needle valve for metering fuel between the upperand lower fuel chambers during only an initial increment of upwardmovement of the needle valve from its closed position substantially lessthan said predetermined maximum lift and slightly greater than saidpredetermined intermediate lift, and a corresponding last increment ofdownward movement of the needle valve, thereby to regulate the rate offuel injection during said initial increment of upward movement and thepressure in the lower fuel chamber during said last increment ofdownward movement.
 13. A fuel injection method according to claim 12wherein said initial increment of opening movement is approximately onehalf said predetermined lift.
 14. A fuel injection method according toclaim 12 wherein said initial increment of opening movement is greaterthan said predetermined intermediate lift in the range of 0.001 to 0.005inch.
 15. A fuel injection method according to claim 12 wherein saidinitial increment of opening movement is in the range of 0.004 to 0.008inch.
 16. A fuel injection method according to claim 12 wherein saidpredetermined maximum lift is in the range of 0.008 to 0.016 inch.
 17. Afuel injection method according to claim 12 wherein said meteringpassage is provided by an annular clearance passageway between the lowerrings of the nozzle body and needle valve having a diametral clearancein the range of 0.0003 to 0.0006 inch.
 18. A fuel injection methodaccording to claim 12 wherein the lower ring of the needle valve has adiameter greater than that of the valve seat and less than that of theupper guide of the needle valve. .Iadd.
 19. A fuel injector comprising anozzle body with an elongated valve bore, an annular valve seat andlongitudinally spaced, coaxial, upper valve guide and lower outer valvering above the valve seat; an elongated nozzle needle valve in the valvebore having longitudinally spaced, coaxial, upper guide and lower innerring which cooperate with the valve guide and outer ring respectively ofthe nozzle body to provide axial movement of the needle valve within thevalve bore between a lower closed position in engagement with the valveseat and an upper fully open position with a predetermined maximum lift;the nozzle body having hole means connected to the valve bore below thevalve seat for injection of fuel; the nozzle body providing an upperfuel chamber surrounding the needle valve between the upper valve guideand outer ring and a lower fuel chamber surrounding the needle valvebetween the outer ring and valve seat; two stage valve closure springmeans biasing the needle valve downwardly into engagement with the valveseat, including first stage spring means holding the needle valve in itsclosed position and biasing the needle valve downwardly as it is liftedupwardly from its closed to its fully open position and second stagespring means biasing the needle valve downwardly as it is liftedupwardly from a predetermined intermediate position with a predeterminedintermediate lift to its fully open position; the upper guide of theneedle valve having a diameter greater than both the inner ring andvalve seat and the inner ring having a diameter greater than the valveseat to provide two differential areas for hydraulically opening theneedle valve against the bias of the spring means; the upper fuelchamber being connected to receive periodic high pressure pulses of fuelfor opening the needle valve against the bias of the spring means andfor supplying fuel for fuel injection through the hole means; the outerrinq forming an outer metering ring with an internal, annular meteringsurface; the inner ring forming an inner metering ring with an externalannular, metering surface; the inner metering ring, with the needlevalve in its closed position, being received within the outer meteringring with a predetermined axial overlap greater than said predeterminedintermediate lift and with a predetermined clearance between the innerand outer metering surfaces providing a metering passageway to regulatefuel flow between the upper and lower fuel chambers during predeterminedopening movement of the needle valve from its closed position greaterthan said intermediate lift and to regulate the pressure in the lowerfuel chamber durinq the last part of the closing movement of the needlevalve..Iaddend. .Iadd.20. A fuel injector according to claim 19 whereinthe fuel injector is a hole type injector in which the nozzle body has anozzle tip below the needle valve enclosing the lower end of the valvebore and said hole means comprises one or more spray holes connected tothe valve bore below the valve seat..Iaddend. .Iadd.21. A fuel injectoraccording to claim 19 wherein said clearance is an annular clearancebetween the inner and outer metering surfaces. .Iadd.22. A fuel injectoraccording to claim 19 wherein said axial overlap is.greater than saidpredetermined intermediate lift in the range of 0.001 to 0.005inch..Iaddend. .Iadd.23. A fuel injector according to claim 21 whereinsaid annular clearance is a diametrical clearance in the range of 0.0003to 0.0006 inch in the closed position of the needle valve. .Iadd.24. Afuel injector according to claim 19 wherein said axial overlap is nogreater than approximately 0.008 inch. .Iadd.25. A fuel injectoraccording to claim 19 wherein the inner ring metering surface iscylindrical. .Iadd.26. A fuel injector according to claim 19 wherein theouter ring metering surface is cylindrical. .Iadd.27. A method of fuelinjection with a fuel injector comprising a nozzle body with anelongated valve bore, an annular valve seat and longitudinally spaced,coaxial, upper valve guide and lower outer valve ring above the valveseat; an elongated needle valve in the valve bore having longitudinallyspaced, coaxial, upper guide and lower inner ring which cooperate withthe valve guide and outer ring respectively of the nozzle body toprovide axial movement of the needle valve within the valve bore betweena lower closed position in engagement with the valve seat and an upperfully open position having a predetermined maximum lift; the nozzle bodyhaving hole means connected to the valve bore below the valve seat forinjection of fuel; the nozzle body providing an upper fuel chambersurrounding the needle valve between the Upper valve guide and outerring and a lower fuel chamber surrounding the needle valve between theouter ring and valve seat; closure spring means biasing the needle valvedownwardly into engagement with the valve seat, including first stagespring means holding the needle valve in its closed position and biasingthe needle valve downwardly as it is lifted upwardly from its closed toits fully open position and second stage spring means biasing the needlevalve downwardly as it is lifted upwardly from a predeterminedintermediate position with a predetermined intermediate lift to itsfully open position; the upper guide of the needle valve having adiameter greater than both the inner ring and valve seat and the innerring having a diameter greater than the valve seat to provide twodifferential areas for hydraulically opening the needle valve againstthe bias of the spring means; the upper fuel chamber being connected toreceive high pressure pulses of fuel for opening the needle valveagainst the bias of the spring means and for supplying fuel for fuelinjection through the hole means; the method comprising the steps ofproviding predetermined fuel metering.passage between the outer andinner rings of the nozzle body and needle valve for metering fuelbetween the upper and lower fuel chambers during predetermined.openingmovement of the needle valve from its closed position greater than saidpredetermined intermediate lift and during the last part of the closingmovement of the needle valve, thereby to regulate the rate of fuelinjection during said predetermined opening movement and the pressure inthe lower fuel chamber during said last part of closingmovement..Iaddend. .Iadd.28. A fuel injection method according to claim27 wherein said predetermined opening movement is approximately one halfsaid predetermined lift..Iaddend. .Iadd.29. A fuel injection methodaccording to claim 27 wherein said predetermined opening movement isgreater than said predetermined intermediate lift in the range of 0.001to 0.005 inch..Iaddend. .Iadd.30. A fuel injection method according toclaim 27 wherein said predetermined opening movement is in the range of0.004 to 0.008 inch..Iaddend. .Iadd.31. A fuel injection methodaccording to claim 27 wherein said predetermined maximum lift is in therange of 0.008 to 0.016 inch..Iaddend. .Iadd.32. A fuel injection methodaccording to claim 27 wherein said metering passage is provided by anannular clearance passageway between the outer and inner rings of thenozzle body and needle valve having a diametral clearance in the rangeof 0.0003 to 0.0006 inch in the closed position of the needlevalve..Iaddend.